The present invention is directed, in general, to wireless access systems and, more specifically, to a system for coordinating TDD transmission bursts between sectors in a single cell and between cells in a cellular network.
Telecommunications access systems provide for voice, data, and multimedia transport and control between the central office (CO) of the telecommunications service provider and the subscriber (customer) premises. Prior to the mid-1970s, the subscriber was provided phone lines (e.g., voice frequency (VF) pairs) directly from the Class 5 switching equipment located in the central office of the telephone company. In the late 1970s, digital loop carrier (DLC) equipment was added to the telecommunications access architecture. The DLC equipment provided an analog phone interface, voice CODEC, digital data multiplexing, transmission interface, and control and alarm remotely from the central office to cabinets located within business and residential locations for approximately 100 to 2000 phone line interfaces. This distributed access architecture greatly reduced line lengths to the subscriber and resulted in significant savings in both wire installation and maintenance. The reduced line lengths also improved communication performance on the line provided to the subscriber.
By the late 1980s, the limitations of data modem connections over voice frequency (VF) pairs were becoming obvious to both subscribers and telecommunications service providers. ISDN (Integrated Services Digital Network) was introduced to provide universal 128 kbps service in the access network. The subscriber interface is based on 64 kbps digitization of the VF pair for digital multiplexing into high speed digital transmission streams (e.g., T1/T3 lines in North America, E1/E3 lines in Europe). ISDN was a logical extension of the digital network that had evolved throughout the 1980s. The rollout of ISDN in Europe was highly successful. However, the rollout in the United States was not successful, due in part to artificially high tariff costs which greatly inhibited the acceptance of ISDN.
More recently, the explosion of the Internet and deregulation of the telecommunications industry have brought about a broadband revolution characterized by greatly increased demands for both voice and data services and greatly reduced costs due to technological innovation and intense competition in the telecommunications marketplace. To meet these demands, high speed DSL (digital subscriber line) modems and cable modems have been developed and introduced. The DLC architecture was extended to provide remote distributed deployment at the neighborhood cabinet level using DSL access multiplexer (DSLAM) equipment. The increased data rates provided to the subscriber resulted in upgrade DLC/DSLAM transmission interfaces from T1/E1interfaces (1.5/2.0 Mbps) to high speed DS3 and OC3 interfaces. In a similar fashion, the entire telecommunications network backbone has undergone and is undergoing continuous upgrade to wideband optical transmission and switching equipment.
Similarly, wireless access systems have been developed and deployed to provide broadband access to both commercial and residential subscriber premises. Initially, the market for wireless access systems was driven by rural radiotelephony deployed solely to meet the universal service requirements imposed by government (i.e., the local telephone company is required to serve all subscribers regardless of the cost to install service). The cost of providing a wired connection to a small percentage of rural subscribers was high enough to justify the development and expense of small-capacity wireless local loop (WLL) systems.
Deregulation of the local telephone market in the United States (e.g., Telecommunications Act of 1996) and in other countries shifted the focus of fixed wireless access (FWA) systems deployment from rural access to competitive local access in more urbanized areas. In addition, the age and inaccessibility of much of the older wired telephone infrastructure makes FWA systems a cost-effective alternative to installing new, wired infrastructure. Also, it is more economically feasible to install FWA systems in developing countries where the market penetration is limited (i.e., the number and density of users who can afford to pay for services is limited to small percent of the population) and the rollout of wired infrastructure cannot be performed profitably. In either case, broad acceptance of FWA systems requires that the voice and data quality of FWA systems must meet or exceed the performance of wired infrastructure.
Wireless access systems must address a number of unique operational and technical issues including:
1) Relatively high bit error rates (BER) compared to wire line or optical systems; and
2) Transparent operation with network protocols and protocol time constraints for the following protocols:
a) ATM;
b) Class 5 switch interfaces (domestic GR-303 and international V5.2);
c) TCP/IP with quality-of-service QoS for voice over IP (VOIP) (i.e., RTP) and other H.323 media services;
d) Distribution of synchronization of network time out to the subscribers;
3) Increased use of voice, video and/or media compression and concentration of active traffic over the air interface to conserve bandwidth;
4) Switching and routing within the access system to distribute signals from the central office to multiple remote cell sites containing multiple cell sectors and one or more frequencies of operation per sector; and
5) Remote support and debugging of the subscriber equipment, including remote software upgrade and provisioning.
Unlike physical optical or wire systems that operate at bit error rates (BER) of 10xe2x88x9211, wireless access systems have time varying channels that typically provide bit error rates of 10xe2x88x923 to 10xe2x88x926. The wireless physical (PHY) layer interface and the media access control (MAC) layer interface must provide modulation, error correction and ARQ protocol that can detect and, where required, correct or retransmit corrupted data so that the interfaces at the network and at the subscriber site operate at wire line bit error rates.
The wide range of equipment and technology capable of providing either wireline (i.e., cable, DSL, optical) broadband access or wireless broadband access has allowed service providers to match the needs of a subscriber with a suitable broadband access solution. However, in many areas, the cost of cable modem or DSL service is high. Additionally, data rates may be slow or coverage incomplete due to line lengths. In these areas and in areas where the high cost of replacing old telephone equipment or the low density of subscribers makes it economically unfeasible to introduce either DSL or cable modem broadband access, fixed wireless broadband systems offer a viable alternative. Fixed wireless broadband systems use a group of transceiver base stations to cover a region in the same manner as the base stations of a cellular phone system. The base stations of a fixed wireless broadband system transmit forward channel (i.e., downstream) signals in directed beams to fixed location antennas attached to the residences or offices of subscribers. The base stations also receive reverse channel (i.e., upstream) signals transmitted by the broadband access equipment of the subscriber.
A common protocol used in the air interface between the base stations and the subscriber fixed wireless access (FWA) devices is time division duplex transmission. In TDD systems, the same channel is used for both transmitting and receiving. During a downlink time slot, the base stations transmit and the subscriber FWA devices receive. During an uplink time slot, the subscriber FWA devices transmit and the base station receive.
However, the difference between transmitted power and received power at a base station cell site can be on the order of 100 db. For a cell with multiple sectors collocated at the same cell site, if the TDD transmit and receive time slots are not properly synchronized, the system suffers from self interference. While low side lobe antennas and large separation of frequencies can minimize the self interference at the cell, these techniques increase antenna cost and reduce spectral efficiency, since fewer frequencies can be used in a given cell. This problem extends to cell-to-cell interference where uncoordinated transmissions increase the level of interference (carrier-to-interference ratio, C/I) for a receiver.
Therefore, there is a need in the art for time division duplex (TDD) fixed wireless access (FWA) systems that minimize interference between cell sites and between sectors within a single cell site. In particular, there is a need for time division duplex (TDD) fixed wireless access systems that implement improved timing and synchronization circuitry that coordinates the transmission and reception of TDD data bursts in the uplink and downlink time slots. More particularly, there is a need for timing and synchronization circuitry that provides highly accurate synchronization of the uplink and downlink time slots within sectors in a single cell site and between multiple cells sites in order to reduce both self interference and cell-to-cell interference.
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in a wireless access network, a timing apparatus that coordinates all TDD modems and sectors in a given cell (including multiple frequencies per sector). The present invention also provides a distributed timing source to coordinate timing between cells. More particularly, the present invention comprises a remote RF modem shelf that distributes a timing pulse (e.g., one second) via a system timing distribution structure to all the RF modems in the remote RF modem shelf. The timing source may be internally generated or may be generated by an external source, such as another RF modem shelf or a global positioning system (GPS) source. Where two or more cell sites are used, a GPS timing source at each remote RF modem shelf provides a coordinated one second pulse which is accurate across all cell sites to better than 100 nanoseconds. According to an exemplary embodiment, an integer number of TDD bursts occur over the one second period (e.g., five hundred (500) TDD bursts, each of two millisecond duration, over the one second period).
The management system of the fixed wireless access network provides aggregated measurement of traffic flow for the downlink and uplink. Each RF modem measures demand per sector. Each remote RF modem shelf measures the aggregate for a particular cell site. An access processor that controls a number of remote RF modem shelves measures the aggregate for part of the fixed wireless access network. Finally, multiple access processors that cover a defined geographical area may aggregate measurements through the management system of the fixed wireless access network. The management system may send TDD duration coordination messages to all remote RF modem shelves through a hierarchical management capability.
Accordingly, it is a primary object of the present invention to provide, for use in a first radio frequency (RF) modem shelf associated with a first one of a plurality of base stations in a fixed wireless access network, a timing distribution apparatus for synchronizing the operations of a plurality of RF modems in the first RF modem shelf. According to an advantageous embodiment of the present invention, the timing distribution apparatus comprises: a) a primary master clock source capable of generating an internal master clock signal and an internal master framing signal; b) a plurality of interface control processors, each of the interface control processors capable of receiving the internal master clock signal and the internal master framing signal and capable of using the internal master clock signal and the internal master framing signal to synchronize a downlink transmission of one of the plurality of RF modems; c) a synchronization bus capable of distributing the internal master clock signal and the internal master framing signal from the primary master clock source to the plurality of interface control processors; d) an external interface input port capable of receiving at least one external master synchronization signal from a second RF modem shelf in the fixed wireless access network; and e) an external interface output port capable of receiving at least one of the internal master clock signal and the internal master framing signal from the primary master clock source and transmitting the at least one of the internal master clock signal and the internal master framing signal to the second RF modem shelf.
According to one embodiment of the present inventions the timing distribution apparatus further comprises a secondary master clock source capable of generating the internal master clock signal and the internal master framing signal.
According to another embodiment of the present invention, the synchronization bus comprises a primary timing bus capable of distributing the internal master clock signal and the internal master framing signal from the primary master clock source and a secondary timing bus capable of distributing the internal master clock signal and the internal master framing signal from the secondary master clock source.
According to still another embodiment of the present invention, the primary master clock source generates the internal master clock signal and the internal master framing signal from the at least one external master synchronization signal.
According to yet another embodiment of the present invention, the secondary master clock source generates the internal master clock signal and the internal master framing signal from the at least one external master synchronization signal.
According to a further embodiment of the present invention, the external interface input port is further capable of receiving a building integrated timing supply (BITS) reference signal from a locally disposed external clock source.
According to a still further embodiment of the present invention, the primary master clock source generates the internal master clock signal and the internal master framing signal from the BITS reference signal.
According to a yet further embodiment of the present invention, the secondary master clock source generates the internal master clock signal and the internal master framing signal from the BITS reference signal.
In one embodiment of the present invention, the external interface input port is further capable of receiving a global positioning system (GPS) reference signal from an external GPS clock source.
In another embodiment of the present invention, the primary master clock source generates the internal master clock signal and the internal master framing signal from the GPS reference signal.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation; the term xe2x80x9cor,xe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.